Electrodeposited metals and alloys are customarily used to fabricate microparts patterned with electrodeposited metals and alloys, using processes such as LIGA, and other commonly known patterning techniques. The production of micro-scale metal parts via LIGA (German acronym for lithography, electroplating, and molding) is a multi-step process requiring mask production, synchrotron exposure of the polymethylmethacrylate (PMMA) substrate (typically PMMA bonded to a metallized silicon wafer or a solid metal plate), development of the PMMA, electroplating to fill the cavities left within the PMMA mold, lapping, and final dissolution of the remaining PMMA. This technology is described in U.S. Pat. No. 5,378,583.
In order for the microparts to have proper mechanical functionality in the microsystems in which they are often used, e.g., sensing or actuating devices, the electrodeposited materials must have high strength (≧80 MPa) and display good ductility. Ideally, these materials should possess through-thickness uniformity (of microstructure and composition) and retention of ductility after thermal exposure. Furthermore, the plating process should be performable at or near room temperature. Unique to microparts patterned using the LIGA process, is the requirement that the above properties and material characteristics be realized in high aspect ratio (>10), thick-section deposits (200 μm to 2 mm).
Various electrodeposited elemental metal or alloy thick films that are known in the art may meet some, but not all, of the criteria indicated above. For example, nickel-cobalt alloys are readily electrodeposited and exhibit high yield strengths. However, as cobalt deposition rates are highly dependent on local mass transport conditions, electrodeposition through a thick mold results in compositional nonuniformities, such as non-uniform cobalt concentration. These nonuniformities result in significant variations in the hardness and strength of the micropart, which renders the micropart useless for structural applications typical of microsystem designs. While variations in cobalt concentration may be minimized using extremely low average deposition rates, the use of lower deposition rates results in impractically long electrodeposition times and intractably high film stresses.
Another way to achieve high-strength electrodeposited material is by using additives such as saccharin in the plating bath to produce fine grain sized nickel. However, the addition of saccharin results in the incorporation of sulfur in the electrodeposited nickel. Sulfur concentrations of hundreds of wt. ppm are produced, even at the lowest practical saccharin additions to the bath. While the resulting electrodeposit possesses high strength, good ductility, and good through-thickness compositional and property uniformity, the high sulfur content renders it susceptible to catastrophic embrittlement upon exposure to even modestly elevated temperatures. As a result, sulfur-containing electrodeposited materials may not be used for any application in which temperature excursions of 200° C. may occur.
Nickel-manganese alloys, are described by W. B. Stephenson Jr. (1966) Plating, 53 (2):183. The principal difficulty with these alloys is that under DC plating conditions, high residual stresses develop as the thickness of the deposit increases. These stresses often lead to delamination of the deposited film from the deposition substrate. At a minimum, one of the consequences of this delamination is the failure of the part to deposit satisfactorily. More importantly, such delamination is likely to cause the failure of the entire deposition process, through either the stress-induced failure of the substrate or the inability to planarize the deposited parts to a final target thickness. Further, these stresses increase rapidly as the manganese concentration in the deposit increases. Even a few tenths of a percent increase in manganese concentration have been shown sufficient to cause stresses in excess of 100 MPa.
There is, therefore, a need in the art for a technique that provides an electrodeposited material having the requisite high strength, good ductility, good through-thickness composition, resistance to high temperature embrittlement, property uniformity and low plating stress. The present invention addresses this need.